1. Field of the Invention
This invention relates generally to an image sensor. In particular, it relates to an image sensor with optical guard rings to prevent cross-talk between adjacent pixels.
2. Description of the Related Art
Solid state image sensors are necessary components in many optoelectronic devices, including digital cameras, cellular phones, and toys. Conventional solid-state image sensors for color analog or digital video cameras are typically charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) photodiode array structures which comprise a spectrally photosensitive layer below one or more layers patterned in an array of color filters and above which resides a surface-layer array of microlens elements. The elementary unit of the image sensor is defined as a pixel. The basic technology used to form the CMOS image sensor is common to both types sensors.
The CMOS image sensor comprises a photo detector for detecting light and a logic circuit for converting the detected light into an electric signal representing data regarding the detected light. The fill factor, sometimes referred to as the aperture efficiency, is the ratio of the size of the light-sensitive area to the size of the total pixel size. Although efforts have been made to increase the fill factor of the image sensor and thereby increase the sensor sensitivity, further increases in the fill factor are limited because the associated logic circuitry cannot be completely removed. Accordingly, in order to increase the sensitivity of the light, a microlens formation technology has been used to converge and focus the incident light onto the photo detector by changing the path of the light that reaches the lens of the photo detector. In order for the image sensor to detect and provide a color image, it typically must include both a photo detector for receiving the light and generating and accumulating charge carriers and a color filter array (CFA), i.e., a plurality of color filter units sequentially arranged above the photo detector. The CFA typically uses one of two alternative three-color primary configurations, either red R, green G and blue B (RGB) or yellow Y, magenta M and cyan C (CMY). A plurality of microlenses are positioned above the color filter array to increase the photosensitivity of the image sensor.
FIG. 1 shows a traditional image sensor disposed in the substrate. The incident light 30 may not effectively focus on the photodiode 12 and may transmit to the adjacent photodiode 12′. As the pixel size is shrunk and multi-layer metal is used to reduce sensor cost, the cross-talk issue is more serious due to light scattering coming from metal layers 16 and 20. Thus, image resolution is degraded, especially for black and white sensors, and color correction is more difficult for color sensors.